Undesired emergency brake applications have long been a source of concern to railroads. While many causes of undesired emergencies have been suggested, randomly occurring brake pipe pressure pulsations have now been identified as a contributing cause of undesired emergencies. These short term, relatively high rate pressure fluctuations seem to be due mainly to the dynamic effects of slack run-in and run-out during over-the-road train operation. Tests have shown that during this slack action, bending of the hose between cars can occur, which produces momentary volumetric changes in the brake pipe and consequent pressure pulses. In addition, the fact that the mass of air in the brake pipe obeys Newton's laws of motion, a general pressure drop of approximately 0.4 psi. has been measured at the rear of a train during slack run-ins, because the air in motion tends to remain in motion; and during slack run-outs, a general pressure rise has been measured at the rear of the train. The magnitude of these pressure fluctuations varies in proportion to the intensity of the slack action and other train parameters. When combined with quick service activity during service brake applications, these pressure fluctuations may generate a momentary localized brake pipe pressure reduction at an emergency rate or at a rate that so closely approaches an emergency rate that a particularly sensitive control valve will respond to trigger an undesired emergency brake application, as will hereinafter be explained.
The emergency piston in the well-known, industry standard ABD/ABDW control valves operates a slide valve that makes "breather" port connections through which quick action chamber pressure on one side of the emergency piston vents to atmosphere via a "breather" choke when the brake pipe pressure effective on the opposite side of the emergency piston is reduced. The "breather" choke is selected to establish a maximum rate at which the fixed volume quick action chamber pressure is capable of venting. By setting this rate in accordance with a threshold rate of reduction of brake pipe pressure, above which it is desired to trigger an emergency application, a pressure differential is prevented from developing across the emergency piston for a duration sufficient to force the emergency piston to emergency position during service rates of reduction of brake pipe pressure. Only when the reduction of brake pipe pressure exceeds such a threshold rate will it be sufficient to develop and sustain a pressure differential across the emergency piston, needed to force the emergency piston to emergency position.
In FIG. 1 is shown the emergency portion 1 of the standard ABD/ABDW control valve device having an emergency piston 2 that carries a slide valve 3 that, in turn, establishes the aforementioned "breather" connection between a port v in the slide valve with a port v2 in the slide valve bushing 5. The orifice area of this v/v2 porting connection initially increases as piston 2 moves from point (a) to point (b'), as shown in the graph of FIG. 2 and decreases as piston 2 moves from point (c') to point (d). Point Y on the graph of FIG. 2 represents the orifice area of the "breather" choke 23. It will be apparent from the graph that the effective orifice area of the v/v2 porting corresponds with the fixed orifice area of "breather" choke 23 when piston travel reaches point (b), being less than the orifice area of "breather" choke 23 between points (a) and (b) and greater between points (b) and (c). In the range of piston travel between points (b) and (c), "breather" choke 23 is effective in conjunction with the v/v2 orifice area to limit the aforementioned maximum rate at which the quick action chamber pressure can vent or "breath", this range of travel being commonly referred to as a maximum "breathing" range.
During piston movement beyond point (c') to point (d), the area of the v/v2 porting interface diminishes as port v moves out of communication with port v2, until at point (d) complete cut-off of the venting or "breathing" of quick action chamber pressure occurs. This range of travel of the emergency piston between points (a) and (d) is generally referred to as the "breathing" zone and lies intermediate release position and emergency position of the emergency piston. The piston travel in this "breathing" zone is nominally, 0.077 inch.
It will be appreciated that since the orifice areas of the v/v2 port connection and "breather" choke 23 are in series between the quick action chamber and atmosphere, the venting or "breathing" of quick action chamber pressure is nearly always influenced by this restriction of both orifices. It will be appreciated, therefore, that the actual "breathing" rate of quick action chamber pressure only approximates the curve of FIG. 2.
Movement of emergency piston 2 into the service zone, in response to a pressure differential created by a reduction in brake pipe pressure relative to quick action chamber pressure acting on opposite sides of piston 2, is intended to vent quick action chamber pressure at a "breathing" rate sufficient to counteract the reduction of brake pipe pressure and thereby reverse the pressure differential initiating piston movement to accordingly stabilize the emergency piston, provided the reduction of brake pipe pressure is at a service rate. If the service reduction of brake pipe pressure is at the maximum service rate, the emergency piston will find a position generally in the maximum "breathing" range between points (b) and (c), in which the resultant venting of quick action chamber pressure will counteract the reduction of brake pipe pressure and thereby stabilize the emergency piston. On the other hand, brake pipe pressure reductions at less than the maximum service rate will create a lower initial pressure differential to govern movement of the emergency piston, and the emergency piston will accordingly find a position in the "breathing" zone between points (a) and (b), where the quick action chamber pressure is vented at a less than maximum "breathing" rate, depending upon the degree of v/v2 interface opening. During this stabilization of the emergency piston during service brake applications, it will be understood that the service piston may cycle within the "breathing" zone until it finds the proper position in which the "breathing" of the quick action chamber pressure balances the brake pipe pressure reduction sufficiently to stabilize the piston. It will also be understood that in the event the emergency piston moves beyond position (c), the v/v2 interface opening begins to gradually close, as port v in the slide valve moves past port v2 in the slide valve bushing seat, thereby tending to decrease the venting of quick action chamber pressure. Accordingly, the pressure differential across the emergency piston effecting its movement may not be reversed, but rather may increase due to the decreased rate of quick action chamber venting, thereby forcing the emergency piston to emergency position, wherein the emergency piston slide valve establishes a port connection to initiate the emergency brake application function. While this is a normal required function in response to a true emergency rate of reduction of brake pipe pressure, it is also believed to occur in response to the above discussed random fluctuations of brake pipe pressure during service brake applications to cause unintended emergency brake applications, as hereinafter explained.
The force of the pressure differential required to overcome static friction and initiate movement of the emergency piston from release position to the "breathing" zone is greater than the force required to overcome dynamic friction and maintain continued movement of the piston. The piston momentum, if excessive, due to this initial high differential force and an extremely fast initial rate of brake pipe pressure reduction, can tend to drive the piston beyond the maximum "breathing" range between points (b) and (c), thus aggravating the situation in which a random brake pipe pressure fluctuation occurs. Normally, piston movement will be halted in the "breathing" zone between points (a) and (c) and will hunt or cycle to find the precise position in which the quick action chamber pressure will "breathe" at a rate corresponding to the effective service reduction of brake pipe pressure at that particular valve, thereby stabilizing the piston against further movement beyond point (c). However, if a random brake pipe pressure fluctuation should occur prior to piston movement being halted, the piston momentum coupled with the momentary high pressure differential acting on the piston, due to the pressure fluctuation, can cause the piston to overshoot the maximum "breathing" range (b-c) within the "breathing" zone, before the pressure fluctuation dissipates. The resultant movement of the emergency piston beyond point (c), defining the limit of the maximum "breathing" range, results in progressively reduced "breathing" of quick action chamber pressure, due to reduced orifice area of the v/v2 porting, with consequent loss of the emergency piston stability. When this occurs, the emergency piston will continue to move to emergency position, producing an unintended emergency brake application.
One way of solving this problem would be to decrease the overall emergency piston sensitivity, but this could severely jeopardize the propagation of emergency brake applications, particularly where successive cars are hauled in a train with inoperative control valves and no other means to effect a local emergency venting of brake pipe pressure in response to an emergency brake application.